Process for producing sandwich-shaped synthetic resin-molded article

ABSTRACT

In producing a sandwich-shaped synthetic resin-molded article of a high quality comprised of a core and an outer layer covering the core, the following steps are used: a first step of injecting an outer layer forming material into a forming cavity in a mold through a gate at an injecting speed V 1 , a second step including an injecting stage in which an injection speed V 2  of the core forming material is set to be higher than the injection speed V 1  at the first step, in order to inject a core forming material under the injection of the outer layer forming material to flow into the outer layer forming material present within the gate and the cavity and to allow the outer layer and core forming materials to flow within the cavity, and a third step of injecting said outer layer forming material at an injection speed V 3  equal to or lower than the final injection speed of the outer layer forming material at the second step, and pushing the core and outer layer forming materials having a double structure existing within the gate into the cavity, thereby forming the core and the outer layer.

FIELD OF THE INVENTION

The present invention relates to an improvement in a process forproducing a sandwich-shaped synthetic resin-molded article, and inparticular, to a process for producing a sandwich-shaped syntheticresin-molded article comprised of a core and an outer layer which coversthe core, comprising a first step of injecting an outer layer formingmaterial into a forming cavity in a mold through a gate, a second stepof injecting a core forming material to flow into the outer layerforming material present within the gate and the cavity and to allow theouter layer and core forming materials to flow within the cavity, and athird step of pushing the core and outer layer forming materials havinga double structure existing within the gate into the cavity by the outerlayer forming material, thereby molding the core and the outer layer.

BACKGROUND ART

There is such a conventionally known producing process disclosed inJapanese Patent Application Laid-open No.6-328509. In this case, themaximum value of an injection speed V₂ of the core forming material atthe second step is set to be lower than the maximum value of aninjection speed V₁ of the outer layer forming material at the firststep.

In a producing process of such type, while the outer layer formingmaterial is flowing within the cavity, the core forming material isallowed to flow into the outer layer forming material. However, after afront portion of the outer layer forming material in a flowing directionreaches a terminal end of the cavity to stop its flowing, it isdifficult to allow the core forming material to flow into the frontportion of the outer layer forming material in the flowing direction.

If the injection speed is set as in the prior art under such acircumstance, the following problem is liable to arise: The frontportion of the core forming material in the flowing direction has notyet flown into the front portion of the outer layer forming material inthe flowing direction at a time point when the front portion of theouter layer forming material in the flowing direction reaches near theterminal end of the cavity. Due to this, it is impossible to allow thecore forming material to flow into the front portion of the outer layerforming material in the flowing direction and hence, it is impossible toproduce a synthetic resin-molded article having a sandwich structureover the whole thereof.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a producing processof the above-described type, wherein the core forming material can bereliably allowed to flow into the front portion of the outer layerforming material in the flowing direction, thereby producing a syntheticresin-molded article having a sandwich structure over the whole thereof.

To achieve the above object, according to the present invention, thereis provided a process for producing a sandwich-shaped syntheticresin-molded article comprised of a core and an outer layer which coversthe core, the process comprising a first step of injecting an outerlayer forming material into a forming cavity in a mold through a gate atan injecting speed V₁, a second step of injecting a core formingmaterial at an injection speed V₂ higher than the injection speed V₁,while injecting the outer layer forming material at the injection speedV₁, in order to allow the core forming material to flow into the outerlayer forming material present within the gate and the cavity and toallow the outer layer and core forming materials to flow within thecavity, the injection speed of the outer layer forming material beingthen reduced to be lower than the injection speed V₁, while theinjection speed V₂ is maintained, and a third step of injecting theouter layer forming material at an injection speed V₃ equal to or lowerthan a final injection speed of the outer layer forming material at thesecond step, and pushing the core and outer layer forming materialshaving a double structure and existing within the gate into the cavity,thereby molding the core and the outer layer.

If the injection speed V₂ of the core forming material at the secondstep is set to be higher than the injection speed V₁ of the outer layerforming material at the first step, as described above, the fillingefficiency of the core forming material can be enhanced. Thus, at a timepoint when a front portion of the outer layer forming material in aflowing direction reaches near a terminal end of the cavity, a frontportion of the core forming material in a flowing direction can beallowed to exist within such front portion of the outer layer formingmaterial in the flowing direction.

If the injection speed V₁ of the outer layer forming material is reducedto relatively further increase the injection speed V₂ of the coreforming material before the start of the second step, there is apossibility that the following disadvantage may arise: the thickness ofthe outer layer is reduced due to the rapid flowing of the core formingmaterial into the outer layer forming material. This is liable to occurwhen a hot runner is used.

In the present invention, at an initial stage of the start of the secondstep, the injection speed V₁ of the outer layer forming material ismaintained at the same level as that at the first step and hence, thedisadvantage as described above is reliably avoided.

Thereafter, the injection speed of the outer layer forming material iscontrolled to be lower than the injection speed V₁ and hence, theinjection speed V₂ of the core forming material is relatively furtherincreased. Thus, it is possible to enhance the filling efficiency of thecore forming material and in its turn, to shorten the cycle time for theproduction of the molded article.

If the injection speed V₃ of the outer layer forming material at thethird step is set to be equal to or lower than the final injection speedof the outer layer forming material at the second step, the outer layerforming material cannot flow into the core forming material at the thirdstep, whereby portions of the synthetic resin-molded articlecommunicating with the gate can be formed by only the outer layerforming material.

In this manner, it is possible to produce a synthetic resin-moldedarticle of a high quality having a sandwich structure over the wholethereof and to enhance the mass productivity of the molded article.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a bumper;

FIG. 2 is a sectional view taken along a line 2--2 in FIG. 1;

FIG. 3 is a longitudinal sectional view of an injection moldingapparatus;

FIG. 4 is a view for explaining a first step;

FIG. 5 is a view for explaining a second step;

FIG. 6 is a view for explaining a third step;

FIG. 7 is a graph illustrating the relationship between the injectiontime and the injection speed in an embodiment;

FIG. 8 is a graph illustrating the relationship between the injectiontime and the injection speed in a first comparative example;

FIG. 9 is a graph illustrating the relationship between the injectiontime and the injection speed in a second comparative example; and

FIG. 10 is a graph illustrating the relationship between the injectiontime and the injection speed in a third comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a bumper 3 made of a synthetic resin as asandwich-shaped synthetic resin-molded article is mounted at a frontportion of a vehicle body 2 of an automobile vehicle 1. The bumper 3 iscomprised of a core 4 and an outer layer 5 which covers the core 4, asshown in FIG. 2.

A main material for forming the core 4 is a material resulting from thepulverization of a recovered bumper which has been formed using apolypropylene (PP) modified with an ethylene propylene rubber (EPR) andwhich has a thermosetting synthetic resin coating film on a surfacethereof. A main material for forming the outer layer 5 is apolypropylene modified with an ethylene-propylene rubber.

Referring to FIG. 3, an injection molding apparatus 6 used for producingthe bumper 3 includes a first injection unit 8 having a screw 7 toinject an outer layer forming material, a second injection unit 10having a screw 9 to inject a core forming material, a nozzle 11 whichconnects both the injection units 8 and 10 to each other, a hot runnerblock 12 connected to the nozzle 11, and a split mold 13 attachable toand detachable from the hot runner block 12. The mold 13 is comprised ofa stationary die 13₁ on the side of the hot runner block 12, and aplurality of movable dies 13₂ movable toward and away from thestationary die 13₁.

A supply passage 14 in the nozzle 11 communicates with a plurality ofgates 16 in the mold 13 through a hot runner 15. Each of the gates 16communicates with a bumper forming cavity 17. In FIG. 3, H₁ is a bandheater, and H₂ is a cartridge heater.

A hollow outer needle 18 and a solid inner needle 19 are concentricallydisposed within the nozzle 11. In the outer needle 18, a valve portion20 at a tip end thereof is opposed to the supply passage 14, and apiston 21 at a base end thereof is slidably received in a cylinder 22.The piston 21 and the cylinder 22 constitute a supply passageopening/closing mechanism 23. In the inner needle 19, a valve portion 24at a tip end thereof is opposed to a valve bore 25 defined in the valveportion 20 of the outer needle 18, and a piston 26 at a base end thereofis slidably received in a cylinder portion 27 defined at the base end ofthe outer needle 18. The piston 26 and the cylinder portion 27constitute a valve bore opening/closing mechanism 28.

The outer needle 18 has a tapered outer peripheral surface connected tothe valve portion 20 thereof, and an outer passage 29 is defined betweensuch tapered outer peripheral surface and an inner peripheral surface ofthe nozzle 11. The outer passage 29 is adapted to communicate at one endthereof with the supply passage 14 and at the other end thereof with thefirst injection unit 8 via a through-bore 30 in the nozzle 11. The outerneedle 18 has a straight outer peripheral surface at its base end, sothat the through-bore 30 can be closed by the straight outer peripheralsurface.

The inner needle 19 has a tapered outer peripheral surface connected tothe valve portion 24 thereof, and an inner passage 31 is defined betweensuch tapered outer peripheral surface and an inner peripheral surface ofthe outer needle 18. The inner passage 31 is adapted to communicate atone end thereof with valve bore 25 and at the other end thereof with thesecond injection unit 10 via through-bores 32 and 33 in the outer needle18 and the nozzle 11. The inner needle 19 has a straight outerperipheral surface at its base end, so that the through-bore 32 can beclosed by such outer peripheral surface.

A particular example of the production of the bumper 3 using theinjection molding apparatus 6 will be described below.

A. Preparation of Core Forming Material

A bumper formed in a molding manner using a polypropylene modified withan ethylene-propylene rubber and having a two-pack urethane coating filmon a surface was selected as the recovered bumper. The composition ofthis recovered bumper was as follows:

    ______________________________________                                        Polypropylene         64% by weight                                             Ethylene-propylene rubber 28% by weight                                       Talc  7% by weight                                                            Coating film  1% by weight                                                  ______________________________________                                    

The recovered bumper was thrown into a pulverizing machine, where it waspulverized. Then, the pulverized material was thrown into a granulator,where it was subjected to a melt-kneading, and the melt-kneaded mass waspassed through a filter of 80 to 100 meshes to remove large grains.Thereafter, the melt-kneaded mass was passed through a fine tube havingan inside diameter of about 1 mm to form a linear material. The linearmaterial was cut into a length of 1 mm to several mm to provide apellet-shaped core forming material.

B. Preparation of Outer Layer Forming Material

To provide the same composition as the composition of the recoveredbumper (but excluding the coating film), a mixture comprising

    ______________________________________                                        Polypropylene         63% by weight                                             Ethylene-propylene rubber 30% by weight                                       Talc  7% by weight                                                          ______________________________________                                    

was thrown into the granulator to produce a pellet-shaped outer layerforming material in a manner similar to that described in the item A.

C. Production of Bumper

(i) In FIG. 3, the outer layer forming material was placed into thefirst injection unit 8 and maintained in a molten state at 210° C., andthe core forming material was placed into the second injection unit 10and maintained in a molten state at 200° C. The stationary die 13₁ andthe movable dies 13₂ were preheated to 40° C. and 50° C., respectively.

(ii) At a first step, the outer layer forming material M₁ was injectedinto the bumper forming cavity 17 in the mold 13 through the supplypassage 14, the hot runner 15 and the gates 16 under the operation ofthe first injection unit 8 with the outer passage 29 being in acommunicating state and with the inner passage 31 being in a cut-offstate, as shown in FIG. 4.

(iii) At a second step, as shown in FIG. 5, a portion of the outerpassage 29 on the side of the supply passage 14 was throttled by thevalve portion 20 under the operation of the first injection unit 8, andthe core forming material M₂ was injected under the operation of thesecond injection unit 10 with the inner passage 31 being in thecommunicating state, and was allowed to flow into the outer layerforming material M₁ existing within the supply passage 14, the hotrunner 15, the gates 16 and the cavity 17. At the same time, the outerlayer an forming material M₁ and M₂ were allowed to flow within thecavity 17. Then, the operation of the second injection unit 10 wasstopped.

(iv) At a third step, the outer layer and core forming material M₁ andM₂ having a double structure and existing within the supply passage 14,the hot runner 15 and the gates 16 were pushed into the cavity 17 by theouter layer forming material M₁ under the operation of the firstinjection unit 8 with the outer passage 29 being in the communicatingstate and with the inner passage 31 being in the cut -off state, asshown in FIG. 6, thereby forming the core 4 and the outer layer 5. Then,the operation of the first injection unit 8 was stopped.

(v) The mold 13 was separated from the hot runner block 12 and thenopened to provide the bumper 3.

After the above-described producing process, the molding of the bumper 3can be subsequently carried out, because the outer layer formingmaterial M₁ has been maintained in the molten state within the hotrunner 15.

Table 1 shows the injection times and the injection speeds for the outerlayer forming material M₁ and the core forming material M₂ at the firstto third steps in the embodiment.

                  TABLE 1                                                         ______________________________________                                        Outer layer forming material                                                                      Core forming material                                     Injection     Injection Injection   Injection                                   speed (mm/sec) time (sec) speed (mm/sec) time (sec)                         ______________________________________                                        At first                                                                             V.sub.1 39.7   6.2     --        --                                      step                                                                        At second                                                                            Va(V.sub.1)                                                                           39.7   1.8     V.sub.2                                                                             48.4  4.0                                   step Vb 19.5 1.4                                                               Vc 9.7 0.8                                                                 At third                                                                             V.sub.3 (Vc)                                                                          9.7    1.7     --        --                                      step                                                                        ______________________________________                                    

FIG. 7 is a graph taken based on Table 1 and illustrating therelationship between the injection time and the injection speed. In thiscase, the injection speed was controlled at one stage for the outerlayer forming material M₁ at the first step; at one stage for the coreforming material M₂ and at three stages for the outer layer formingmaterial M₁ at the second step; and at one stage for the outer layerforming material M₁ at the third step.

More specifically, at the first step, the outer layer forming materialM₁ was injected at the injection speed V₁ (Va) into the forming cavity17 in the mold 13 through the gates 16. At the second step, the coreforming material M₂ was injected at the injection speed V₂ higher thanthe injection speed V₁, while injecting the outer layer forming materialM₁ at the injection speed V₁, so that the core forming material M₂ wasallowed to flow into the outer layer forming material M₁ existing withinthe gates 16 and the cavity 17, and at the same time, the outer layerand core forming materials M₁ and M₂ were allowed to flow within thecavity 17. Then, the injection speeds Vb and Vc of the outer layerforming material M₁ were reduced to be lower than the injection speedV₁, while maintaining the injection speed V₂. At the third step, theouter layer forming material M₁ was injected at the injection speed V₃(Vc) equal to or lower than the final injection speed of the outer layerforming material M₁ at the second step, and the outer layer and coreforming materials M₁ and M₂ having a double structure and existingwithin the gates 16 were pushed into the cavity 17, thereby molding thecore 4 and the outer layer 5.

If the injection speed V₂ of the core forming material M₂ is set at thesecond step so as to be higher than the injection speed V₁ of the outerlayer forming material M₁ at the first step, as described above, thefilling efficiency of the core forming material M₂ can be enhanced.Thus, at a time point when a front portion of the outer forming materialM₁ in a flowing direction reaches near a terminal end of the cavity 17,a front portion of the core forming material M₂ in a flowing directioncan be permitted to exist within such front portion of the outer formingmaterial M₁ in the flowing direction, as shown in FIG. 5.

If the injection speed V₁ of the outer layer forming material M₁ isreduced to relatively further increase the injection speed V₂ of thecore forming material M₂ before the start of the second step, there is apossibility that the following disadvantage may arise: the thickness ofthe outer layer 5 is reduced due to the rapid flowing of the coreforming material M₂ into the outer layer forming material M₁. This isliable to occur when the hot runner 15 is used.

In this embodiment, at an initial stage of the start of the second step,the injection speed V₁ of the outer layer forming material M₁ ismaintained at the same level as that at the first step and hence, thedisadvantage as described above is reliably avoided.

Thereafter, the injection speeds Vb, Vc of the outer layer formingmaterial M₁ are controlled to be lower than the injection speed V₁ andhence, the injection speed V₂ of the core forming material M₂ isrelatively further increased. Thus, it is possible to enhance thefilling efficiency of the core forming material M₂ and in its turn, toshorten the cycle time for the production of the bumper.

If the injection speed V₃ of the outer layer forming material M₁ at thethird step is set to be equal to or lower than the final injection speedVc of the outer layer forming material M₁ at the second step, e.g., tobe V₃ =Vc in the production of the example 1, the outer layer formingmaterial M₁ cannot flow into the core forming material M₂ at the thirdstep. Thus, portions of the bumper 3 communicating with the gates can beformed by only the outer layer forming material M₁.

In this manner, the example 1 of the bumper 3 having a sandwichstructure over the whole thereof could be produced. In this example 1,the filling rate of the core forming material M₂ prepared from therecovered bumper is high, which is effective for providing a reductionin production cost of the bumper 3 and the resource-saving.

Table 2 shows the injection times and the injection speeds for the outerlayer forming material M₁ and the core forming material M₂ at the firstto third steps in a first comparative example.

                  TABLE 2                                                         ______________________________________                                        Outer layer forming material                                                                      Core forming material                                     Injection      Injection                                                                              Injection   Injection                                   speed (mm/sec) time (sec) speed (mm/sec) time (sec)                         ______________________________________                                        At first                                                                             V.sub.1                                                                              Va    27.1 7.0    --        --                                    step  Vb 26.6 4.1                                                               Vc 17.0 0.3                                                               At second                                                                            Vc     17.0     3.9    V.sub.2                                                                           Vd   39.7 3.9                                 step     Ve 29.0 0.2                                                           Vg 9.7 0.4 -- Vf 19.4 0.2                                                  At third                                                                             V.sub.3                                                                              9.7      1.6    --        --                                      step (Vg)                                                                   ______________________________________                                    

FIG. 8 is a graph taken based on Table 2 and illustrating therelationship between the injection time and the injection speed. In thiscase, the injection speed was controlled at three stages for the outerlayer forming material M₁ at the first step; at three stages for thecore forming material M₂ and at two stages for the outer layer formingmaterial M₁ at the second step; and at one stage for the outer layerforming material M₁ at the third step. Before the start of the secondstep, the injection speed of the outer layer forming material M₁ is setat Vc to be remarkably lower than Vb.

Table 3 shows the injection times and the injection speeds for the outerlayer forming material M₁ and the core forming material M₂ at the firstto third steps in a second comparative example.

                  TABLE 3                                                         ______________________________________                                        Outer layer forming material                                                                      Core forming material                                     Injection      Injection                                                                              Injection   Injection                                   speed (mm/sec) time (sec) speed (mm/sec) time (sec)                         ______________________________________                                        At first                                                                             V.sub.1                                                                              Va    46.0 5.9    --        --                                    step  Vb 24.7 0.5                                                           At second                                                                            Vb     24.7     1.5    V.sub.2                                                                             48.4  3.8                                   step Vc 15.1 1.7                                                               Vd 9.7 0.9                                                                 At third                                                                             V.sub.3                                                                              9.7      1.6    --        --                                      step (Vd)                                                                   ______________________________________                                    

FIG. 9 is a graph taken based on Table 3 and illustrating therelationship between the injection time and the injection speed. In thiscase, the injection speed was controlled at two stages for the outerlayer forming material M₁ at the first step; at one stage for the coreforming material M₂ and at three stages for the outer layer formingmaterial M₁ at the second step; and at one stage for the outer layerforming material M₁ at the third step. Before the start of the secondstep, the injection speed of the outer layer forming material M₁ is setat Vb to be remarkably lower than Va as described above.

In the first and second comparative examples, the injection speed of theouter layer forming material is set before the start of the second stepso as to be remarkably lower than of the preceding step, as describedabove. Therefore, there is a possibility that a reduction in thicknessof the outer layer 5 may be brought about, as described above, but inthe embodiment, there is not such a possibility.

As apparent from FIGS. 7 to 9, the injection time in the embodiment is11.9 seconds, whereas the injection times in the first and secondcomparative examples are 17.3 seconds and 12.1 seconds, respectively.Therefore, in the embodiment, the cycle time for the production of thebumper is shortened, as compared with the first and second comparativeexamples, leading to a good mass productivity.

Table 4 shows the injection times and the injection speeds for the outerlayer forming material M₁ and the core forming material M₂ at the firstand second steps in a third comparative example.

                  TABLE 4                                                         ______________________________________                                        Outer layer forming material                                                                      Core forming material                                     Injection speed                                                                             Injection Injection speed                                                                           Injection                                   (mm/sec) time (sec) (mm/sec) time (sec)                                     ______________________________________                                        At first                                                                             Va     39.7    7.3     --        --                                      step                                                                        At second                                                                            Va     39.7    1.0     Vb    26.6  6.5                                   step Ve 29.2 0.7 Vc 16.9 0.4                                                   Vf 19.4 1.1 Vd  9.7 0.4                                                    ______________________________________                                    

FIG. 10 is a graph taken based on Table 4 and illustrating therelationship between the injection time and the injection speed. In thiscase, the injection speed was controlled at one stage for the outerlayer forming material M₁ at the first step, and at three stages for thecore forming material M₂ and at two stages for the outer layer formingmaterial M₁ at the second step.

Due to the fact that the injection speed Vb of the core forming materialM₂ at the second step was set lower than the injection speed Va of theouter layer forming material M₁ at the first step from the start of theinjection, the following disadvantage was produced: In the case of thebumper in the third comparative example, the core forming material M₂was not permitted to flow into the front portion of the outer layerforming material M₁ in the flowing direction, and the core formingmaterial M₂ broke through the outer layer forming material M₁ at alocation short of such front portion to bite into the surface of theouter layer forming material M₁. In addition, because there is no thirdstep in the case of the third comparative example, it is impossible tocarry out the continuous molding.

In addition to those described above, the outer layer forming materialM₁ used for the production of the bumper 3 includes those describedbelow.

    ______________________________________                                        [Formulation 1]                                                                 Polypropylene 63% by weight                                                   Ethylene-propylene rubber 30% by weight                                       Talc  7% by weight                                                            Weather-proof stabilizer 1 phr                                                UV absorbent 1 phr                                                            Carbon black (pigment) 3 phr                                                  Coating-property improving modifier 3 phr                                     [Formulation 2]                                                               Polypropylene 60% by weight                                                   Ethylene-propylene rubber 30% by weight                                       Talc 10% by weight                                                            Weather-proof stabilizer 1 phr                                                UV absorbent 1 phr                                                            Carbon black (pigment) 3 phr                                                  Coating-property improving modifier 3 phr                                   ______________________________________                                    

In addition to those described above, the core forming material M₂ usedfor the production of the bumper 3 includes those described below.

(1) A forming material comprising 93% by weight of a polypropylene and7% by weight of talc. In this case, the polypropylene includes 30% byweight of an ethylene-propylene based rubber synthesized simultaneouslywith the step of polymerizing the polypropylene. The talc was added tothe synthesized product at the final step of the production of thepolypropylene. This forming material is more inexpensive than a formingmaterial made by separately synthesizing a polypropylene and anethylene-propylene rubber and blending them together.

(2) Forming Material having the following composition:

    ______________________________________                                        Polypropylene     60% by weight                                                 Ethylene-propylene rubber 30% by weight                                       Talc 10% by weight                                                          ______________________________________                                    

It should be noted that the present invention is applied to theproduction of a sandwich-shaped synthetic resin-molded article, e.g.,vehicle parts such as an instrument panel, a console box and the like,and boxes for domestic electric articles.

What is claimed is:
 1. A process for producing a sandwich-shapedsynthetic resin-molded article comprising a core and an outer layerwhich covers said core, said process comprising a first step ofinjecting an outer layer forming material into a forming cavity in amold through a gate at an injecting speed V₁, a second step of injectinga core forming material into said cavity at an injection speed V₂ higherthan the injection speed V₁, while injecting the outer layer formingmaterial at the injection speed V₁, in order to allow the core formingmaterial to flow into the outer layer forming material present withinthe gate and the cavity and to allow the outer layer and core formingmaterials to flow within the cavity, the injection speed of the outerlayer forming material being then reduced to be lower than the injectionspeed V₁, while the injection speed V₂ is maintained, and a third stepof injecting said outer layer forming material at an injection speed V₃equal to or lower than a final injection speed of said outer layerforming material at said second step, and pushing said core and outerlayer forming materials having a double structure existing within saidgate into said cavity, thereby molding the core and the outer layer. 2.A process for producing a sandwich-shaped synthetic resin-molded articleaccording to claim 1, wherein the injection of said core and outer layerforming materials into said gate is conducted through a hot runner.